1. Field of the Invention
The present invention relates to a matrix driving method and apparatus for current-driven display elements such as LED (light emitting diode), ECD (electrochromic display), EL (electroluminescence), and so forth.
2. Description of Related Art
A simple X-Y matrix drive for display elements (will be referred to simply as xe2x80x9cmatrix drivexe2x80x9d hereinunder) has two stripe electrode groups comprising a plurality of scanning electrodes and a plurality of signal electrodes, respectively, disposed perpendicular to each other, and drive circuits connected to the stripe electrodes, respectively, to change a voltage or the like at the intersections, thereby driving display elements disposed at the intersections, respectively.
The matrix drive uses a driving method depending upon a relationship between an input (voltage or current) to the matrix drive and an output from the display element (light intensity, transmittance or reflectance). That is, in case the display element is a liquid crystal, for example, the matrix drive adopts a line-sequential scanning method in which scanning electrodes are selected in a line-sequence, to change the effective voltage applied to the liquid crystal (if the liquid crystal is a TNLC (twisted-nematic liquid crystal) or the voltage polarity (if the liquid crystal is a FLC (ferroelectric liquid crystal)).
On the other hand, current-driven display elements such as LED (light emitting diode), ECD (electrochromic display), EL (electroluminescence), and so forth are driven by a matrix drive shown by way of example in FIG. 1. The matrix drive is generally indicated with a reference 100. As shown in FIG. 1, the matrix drive 100 comprises a set of scanning electrodes ScE (ScE1, ScE2, . . . , ScEy) and a set of signal electrodes SiE (SiE1, SiE2, . . . , SiEx), disposed perpendicular to each other. The above-mentioned current-driven display elements are disposed at intersections of the stripe electrodes in these two sets. The matrix drive 100 further comprises a scanning electrode drive circuit 101 connected to the scanning electrodes ScE and a signal electrode drive circuit 102 connected to the signal electrodes SiE.
As shown in FIG. 1, the scanning electrode drive circuit 101 comprises select switches L (L1, L2, . . . , Ly) connected to the scanning electrodes ScE1, ScE2, . . . , ScEy, respectively. The potential at a selected scanning electrode ScE is dropped to the ground potential (GND) level by turning on or off each of the select switches L by a control signal from a controller (not shown).
On the other hand, the signal electrode drive circuit 102 comprises select switches S (S1, S2, . . . , Sx) connected to the signal electrodes SiE1, SiE2, . . . , SiEx), respectively, and current sources CS (CS1, CS2, . . . , CSx) connected to the select switches S, respectively, and also to a power source 103. By turning on or off the select switches S by a control signal from a controller (not shown), a current is supplied as a display signal to a selected one of the signal electrodes SiE from the current source CS. Thus, as the select switches L and S are turned on or off, the matrix drive 100 line-sequentially drives the current-driven display elements disposed at the intersections of selected scanning electrodes ScE and selected signal electrodes SiE.
In the matrix drive 100, however, there develops a capacitance called xe2x80x9cstray capacitancexe2x80x9d at the intersection of the scanning and signal electrodes ScE and SiE, which causes the following problems.
That is, in the matrix drive 100, when a current (i.e. a display signal) is supplied to the current-driven display elements from the current source CS for line-sequential drive of the display elements, an electric charge will be charged for the stray capacitance. Thus, in the matrix drive 100, a current dedicating to the display does not flow until a threshold voltage Vt required for display (i.e., light emission) of the current-driven display element is reached, so that a xe2x80x9cdead timexe2x80x9d will arise for a time during which one scanning line is selected, as shown in FIG. 2. Therefore, because of the dead time, the matrix drive 100 cannot provide any efficient display for the time for selection of one scanning line. The luminance of the current-driven display element will decrease at this time by a light emitting time/one-scanning line selection timexc3x97100 (%) as will also been seen from FIG. 2.
In the matrix drive 100, the dead time will have a remarkable influence on a gray-scale representation among others. When gray scales are represented at a pulse width ratio of 8:4:2:1, for example, by PWM (pulse width modulation) in the matrix drive 100, the number of gray scales is limited or image quality is deteriorated as shown in FIG. 3 since one scanning line has to be selected for a predetermined time. More specifically, in the matrix drive 100, when a gray scale representation is done within the one scanning line selection time to maintain the pulse width ratio of 8:4:2:1 taking the above-mentioned dead time in consideration, 16 gray scales are reduced to 4 ones, for example, as shown in FIG. 3A, namely, the number of gray scales is insufficient. On the other hand, a gray scale representation is done at the pulse width ratio of 8:4:2:1 by a line-sequential drive taking no account of the dead time, a ratio of 8:4:2:1 in light emitting time cannot correctly be ensured for display times a, b, c and d as shown in FIG. 3B, so that a non-linearization, gamma deterioration, of gray scales will take place and thus gray scale representation cannot correctly be done.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a matrix driving method and apparatus for current-driven display elements, adapted to suppress the influence of a stray capacitance taking place at intersections of scanning and signal electrodes.
The above object can be attained by providing a matrix driving method for current-driven display elements, in which current-driven display elements are disposed, in a matrix fashion, at intersections of a plurality of scanning electrodes and a plurality of signal electrodes, a scanning electrode is selected and a display signal is supplied to each signal electrode to drive each of the current-driven display elements, wherein according to the present invention:
an electric charge is precharged for a capacity of the intersection before the display signal is supplied to the signal electrode.
In the current-driven display element matrix driving method, an electric charge is precharged for the capacity of the intersections, whereby an electric charge is accumulated for the stray capacitance developed at the intersections of the scanning and signal electrodes.
Also, the object can be attained by providing a matrix driving apparatus for current-driven display elements, in which current-driven display elements are disposed, in a matrix fashion, at intersections of a plurality of scanning electrodes and a plurality of signal electrodes, a scanning electrode is selected and a display signal is supplied to each signal electrode to drive each of the current-driven display elements, the matrix driving apparatus comprising, according to the present invention:
means for precharging an electric charge for a capacity of the intersection before the display signal is supplied to the signal electrode.
In the current-driven display element matrix driving apparatus, the precharging means precharges an electric charge for the capacity of the intersections, thereby accumulating an electric charge for the stray capacitance developed at the intersections of the scanning and signal electrodes.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.